1. Field of the Invention
The present invention relates to a fuel injection control device used in a variable-fuel engine which can be operated with an essentially pure alcohol fuel, with a fuel produced by mixing gasoline and alcohol in a variable ratio, or with normal gasoline fuel.
2. Description of the Background Art
Recently, there has been developed an FFV (Flexible Fuel Vehicle) which mounts a variable-fuel engine thereon, wherein the variable-fuel engine is operable either with essentially pure alcohol (ethanol) fuel, with a fuel which is produced by mixing gasoline and alcohol in a variable ratio, or with normal gasoline as a fuel.
Due to the difference in a stoichiometric mixture coefficient with respect to oxygen in combustion compared with gasoline, alcohol, along with the increase of the concentration of alcohol in the fuel, requires the supply of a larger quantity of fuel for the same intake quantity and power output.
Accordingly, in such a variable-fuel engine, a control operation is performed, for correcting a basic fuel injection time in response to the concentration of alcohol in the fuel, in order to obtain an optimum air-fuel ratio (for example, see published patent document JP-A-63-5131).
Here, the concentration of alcohol in the fuel is estimated based on an air-fuel ratio correction coefficient which is obtained using a detection value of an oxygen sensor (hereinafter referred to as “O2 sensor”) which measures the concentration of oxygen contained in an exhaust gas.
However, in the conventional control method, the basic fuel injection time is, for example, obtained by measuring an engine rotary speed and an intake pressure and, thereafter, by retrieving maps based on the measured values of the engine rotary speed and the intake pressure.
Accordingly, even when the concentration of alcohol in the fuel is changed, the basic fuel injection time is the value which is preliminarily set with respect to the engine rotary speed and the intake pressure and hence, there has been a drawback that an adjustment range of a fuel injection quantity with respect to the concentration of alcohol is narrow.
The present invention has been made under such drawbacks, and it is an object of the present invention to provide a fuel injection control apparatus for a variable-fuel engine, which includes a plurality of maps for determining a basic fuel injection time in response to a state of an engine, and changes over the maps in response to the concentration of alcohol.